IGNEOUS ROCKS/Komatiite 265 source, even at low degrees of melting The conditions are: (i) the viscosity of the liquid is low; (ii) the solid matrix is deformable; and (iii) the density of the liquid is less than that of the unmelted residue Although this is the normal situation for melting at low pressures where basaltic magmas are produced, it may not always be the case for komatiites It has been shown through experimental studies that because silicate liquid is more compressible than solid silicate minerals, the density contrast between solid and liquid decreases as pressure increases At pressures greater than about GPa, which correspond to a mantle depth of about 250 km, the density of an ultramafic komatiite liquid exceeds that of olivine It remains less, however, than that of garnet, the densest upper-mantle mineral It has also been shown that at GPa, komatiite magma forms through 30–50% melting, leaving a residue of olivine and majorite garnet The density of the liquid is slightly less than that of the olivine-garnet residue Although under static conditions there would be little impetus for the liquid to segregate from its source, mantle melting is normally due to adiabatic decompression in an ascending source, as in a mantle plume As the source rises, the pressure decreases and the density difference between melt and solid increases Eventually the density contrast becomes sufficiently large that komatiite of the Al-depleted Barberton type escapes from its source, probably in a single batch of high-degree mantle melt This type of komatiite is probably one of the rare types of magma that forms through batch melting of the mantle Figure Al-undepleted or Munro-type komatiites lack the geochemical signature that signals melting in equilibrium with garnet This does not necessarily require that the source was garnet-free; only that when the komatiite magma separated from its source, garnet was absent in the solid residue Three processes can contribute to the elimination of garnet: low pressure, which destabilizes garnet; a high degree of melting, which eliminates low-temperature phases; and fractional melting, which preferentially removes the first-melting garnet-rich component from the source In a rising mantle plume, all three processes may operate together Al-undepleted Munro-type komatiites are characterized by relative depletion of the more incompatible trace elements, such as the light rare-earth elements (LREE), Nb and Th In some cases, notably for the Cretaceous Gorgona komatiites, the extent of LREE depletion varies widely within a suite of rocks of constant Nd isotopic composition This pattern is a Figure Classification of komatiitic flows and sills on the basis of the relative degrees of olivine enrichment and differentiation in situ (Modified from Lesher et al (1984).) UN, undifferentiated non cumulate (massive, pillowed, or volcaniclastic); DN, differentiated non cumulate; UC, undifferentiated cumulate; DC, differentiated cumulate; illustrating the crystallization of a layered spinifex textured komatiite flow